1. Field of the Invention
This invention relates to cellular telephones and more particularly to the selection of vocoder standards for Code Division Multiple Access (CDMA) by mobile telephones.
2. Background Information
Cellular and personal communication service (PCS) mobile telephones are low power radio transmitters. Cellular telephones transmit on frequencies between 824 MHz and 849 MHz, while PCS telephones transmit from 1850 MHz to 1910 MHz. Some units can transmit in both the cellular and PCS frequency bands. Mobile telephone service can be provided by broadcasting either analog or digital information. By broadcasting digital information that encodes analog voice messages, the number of messages that can be transmitted in the allocated frequency range can be increased.
CDMA is one standard for providing mobile telephone service using digital encoding. The CDMA standard is specified for cellular frequencies by the Telecommunications Industry Association (TIA) standard IS-95. CDMA for PCS frequencies is specified by American National Standards Institute (ANSI) standard J-STD-008.
Analog voice information is converted to and from a digital encoding by a vocoder (voice encoder and decoder). The CDMA standards do not specify the type of vocoder to be used. Voice coding is a service option allowing a plurality of vocoders to be used within the CDMA standard. For example, current CDMA systems may use a Qualcomm Code Excited Linear Predictive (Q-CELP) vocoder or an Enhanced Variable Rate Coder (EVRC).
When a mobile telephone user wishes to place a CDMA call, the user enters a telephone number and presses a key, such as "send," to signal the telephone to originate a call. The mobile telephone must then establish contact with a base station on an access channel and request an assignment for a traffic channel. Contact is established by sending an access probe that includes the request message. An access attempt may require that access probes with the same message be transmitted more than once before a response from the base station is received by the mobile telephone. Multiple access probes may be required because the position of the mobile telephone could be changing and the initial access probes may not have been received by a base station due to interference that was present at the time the earlier access probes were transmitted. When the base station receives a recognizable access probe from a mobile telephone, the base station broadcasts an acknowledgement to the mobile telephone. The acknowledgement can accept the call and provide a traffic channel, or reject the call. If the call is rejected, the acknowledgement can include instructions for a directed retry, for example directing the mobile telephone to establish a connection using analog services, or simply reject the call.
Included in the CDMA request message is an indication of the type of vocoder that the mobile telephone will use to encode the voice data. The base station must be capable of encoding and decoding voice data using the requested vocoder to successfully complete the call. If the base station does not recognize the requested vocoder, the base station will reject the call. When a user is in their home area, normally they will use a mobile telephone that broadcasts in a format that is compatible with the base stations of their service provider. However, if the user roams to an area serviced by another provider or if their service provider has a large service area with differing service capabilities within the area, the mobile telephone may attempt to establish a call with a base station that cannot handle the vocoder type requested in the access probe.
Certain mobile telephones are capable of using more than one type of vocoder. This permits the phone to be used in more than one service area where the different service areas do not support the use of the same vocoder. However, if the user roams from one area to another, calls cannot be placed until the user recognizes that a different vocoder is required and appropriate steps are taken to program the mobile telephone to use the correct vocoder. This presents three problems: recognition that the user is in a service area that does not recognize the presently selected vocoder, knowing the type of vocoder that is supported, and setting the mobile phone to use the necessary type of vocoder. Accordingly, there is a need for a method and apparatus for a mobile telephone that will overcome the aforementioned problems.